Higher precision equipment used hitherto for vertical drillings includes equipment which makes use of an inclinometer (typically a triaxial accelerometer) associated with a compass (triaxial magnetometer). Inclinometer and compass are usually contained in a special rod forming a so-called down-hole assembly (or unit). As is known, the inclinometer provides the value of the inclination with respect to the vertical, while the compass indicates the azimuthal angle of the direction containing this inclination.
In order to reposition drilling in its nominal direction, and therefore restore the verticality of the borehole, a deviation must be imparted to the tool in the opposite direction to that of the inclination detected. For this purpose, in order to correct the direction of the drilling, i.e. deviate it, the tool is connected to the drill string by means of a deviation connecting member in the form of an elbow sleeve, referred to in the sector as “bent sub”. The bent sub is arranged between the tool and the string so that the axis of the tool is angularly offset by a few degrees (generally 1 to 3 degrees) with respect to the axis of the drill string.
Hitherto, in order to determine the azimuthal direction in which the so-called tool face must be oriented in order to restore the verticality of the excavation, the information made available by the compass is used. In order to orient correctly the directional tool, it is therefore indispensable for the compass to indicate the right direction. However, the compass does not always function correctly; this may be due to magnetic disturbances induced by metallic bodies or by electric currents flowing in the vicinity of the drilling.
Moreover, with the excavation equipment which is most widely used, the compass may not be permanently contained inside the special tool-holder rod (usually a nonmagnetic stainless-steel rod), but must be lowered to the bottom of the excavation whenever a measurement is performed and then removed in order to start the drilling again. Consequently, the correct orientation of the compass with respect to the bent sub and the boring tool is not always readily obtainable. In particular, on each occasion the compass must be removed and repositioned with great accuracy. The instrument must be locked angularly in a given fixed angular position with respect to the elbow of the bent sub. For this purpose it is necessary to use a guiding and connection device called a “mule shoe” which is lowered inside the down-hole assembly. The mule shoe guides the compass into the correct angular position and prevents it from rotating with respect to the tool bit.
It is indispensable to remove the instrument in particular if boring is performed by means of a hydraulic hammer. This type of tool, which is particularly effective, in fact produces impacts and vibrations which rapidly destroy the compass if it is not removed from the string. Moreover, the high pressures which are required for operation of the hammer may result in the infiltration of water into the data supply/transmission line of the instrument itself.
The orientation of the bent sub in the correct direction may therefore be difficult or, in some situations, even impossible. The precision of the drilling, and therefore the need to correct deviations from the vertical, is of fundamental importance in many applications, for example in the construction of partitions at a depth of more than 40 meters, consisting of posts which are arranged alongside one another and which must overlap by a few centimeters (2-3 cm) in order to ensure the continuity and the impermeability of construction work.
The abovementioned method moreover requires that the compass sensors should be arranged very close to the elbow in order to detect with a high degree of accuracy the inclination and orientation of the bent sub. Owing to this proximity, the compass is affected by the magnetic disturbances of the hammer body. The angular data made available by the compass (through a method known as “magnetic tool face orientation”, MTFO) therefore may not be used during orientation of the tool face. Therefore, although in the calculation of the spatial position of the tool face, the compass error is within acceptable limits and may be corrected by means of several readings, for an evaluation of the orientation of the tool face it is necessary to resort to a method which is not subject to major errors so as to be able to correct the deviation in the shortest possible vertical space and with the maximum efficiency.